1. Field of the Invention
The invention pertains to a frame transfer CCD matrix provided with vertical type anti-blooming means having, on a first type of semiconductor substrate, a second type of channel layer separated by narrow insulating zones into a plurality of columns and surmounted, with an interposed layer of insulating oxide, by a network of transfer gates, extended perpendicularly to the insulating zones and dividing the columns into a large number of "pixels" or elementary picture (or image) elements.
The invention also pertains to a method for the fabrication of a photosensitive matrix such as this.
2. Description of the Prior Art
As is well known, these photosensitive matrices are designed to convert an optical image, projected on the input face of the matrix, into electrical signals formed by a dual sequence of rows and frames. Each frame has a period of accumulation during which the absorbed photons give rise to electron/hole pairs, with one type of charge collecting in elementary cells or pixels while the other type of charge flows through the semiconductor. After the accumulation period, there is a transfer period where the collected charges are sent sequentially, by columns, to conversion capacitors to give rise to a signal voltage. Of course, the transfer period is shorter than the accumulation period.
The cooperation between the insulating zones and the suitably biased transfer gates determines the formation, for each pixel, during the accumulation period, of discrete potential wells separated by walls. The charges collect in these wells which they gradually fill. The filling current or flow is substantially proportionate to the local illumination. During the transfer period, an appropriate modulation of the gate potentials causes the walls of the potential wells to shift towards the columns, towards the conversion capacitors.
It is seen that the charge accumulation capacity of the wells, in conjunction with the frame transfer frequency, determines a range of exploitable illumination levels between a dark level and a blooming level (the latter corresponding to the total filling of the wells) beyond which the charges flow over the walls and fill the adjacent wells.
A standard way to prevent the blooming of zones of the matrix by the overflow of charges from wells into adjacent wells is to have anti-blooming means to drain the extra charges on a barrier level which is close to the blooming level but is lower than it in terms of absolute value. These anti-blooming means are diodes or drains connected to a suitably biased connection so as to be off for a well potential which is higher than the barrier level, and on for a lower potential, in the case where the substrate is of the P type. In the case of a N type substrate, the on and off potentials are respectively lower than and higher than the barrier level.
In the prior art, these anti-blooming means are classified under two types, horizontal or vertical, depending on whether the direction in which the charges flow towards the drains is parallel or perpendicular to the surface of the matrix (which is conventionally considered to stretch along a horizontal plane).
In the horizontal type, as described notably in the documents FR-A-No. 2 529 388 and FR-A-No. 2 529 390, the diodes are placed on the surface of the matrix, adjacent to the pixels. The result thereof is that these diodes, which are optically inactive, occupy a considerable surface of the matrix. The optical aperture of the matrix, or the ratio of the photosensitive surface to the geometrical surface is reduced.
Anti-blooming structures of the vertical type have also been proposed, where the excess charges are discharged through the substrate by means of a buried PN junction formed by a P type recess in an N type substrate. These structures have an optical aperture close to unity. By contrast, they cause a poor spectral response with a loss in sensitivity for the long wavelengths of spectrum, namely red and infra-red. For, the mean distance of absorption of the photons with the formation of electron-hole pairs is relatively great for the low energy photons in silicon, and exceeds the depth of the recesses, this depth being limited, in particular so that the PN junction is close enough to the associated well.
An object of the invention is the making of a photosensitive charge transfer matrix provided with anti-blooming means, and having a wide optical aperture and a good spectral response.